Carbon nanotubes are seamless nanometer scale tubes of graphite sheets with fullerene caps. They have shown promise for nanoscale electronic devices, chemical sensors, high strength materials, field emission arrays, tips for scanning probe microscopy, gas storage, and other important applications.
Carbon nanotubes may be multi-walled or single walled. Carbon multi-walled nanotubes (MWNTs) were discovered in the hard deposit formed on the graphite cathode of an arc-evaporation apparatus used to prepare carbon fullerenes C60 and C70 (S. Iijima, “Helical Microtubules of Graphitic Carbon,” Nature, vol. 354, pp. 56–58 (1991)). Carbon single wall nanotubes (SWNTs) were reported shortly thereafter (Iijima et al., Nature, vol. 363, p. 603, 1993; and Bethune et al., Nature, vol. 363, p. 605, 1993). In the years following their initial discoveries, many other reports relating to carbon nanotubes have appeared. Some of these include the following: Thess, A. et al., Science 273, 483 (1996); Ivanov, V. et al., Chem. Phys. Lett 223, 329 (1994); Li A. et al., Science 274, 1701 (1996); C. Journet et al. Appl. Phys. A, vol. 67, pp. 1–9 (1998); E. G. Rakov, Russ. Chem. Rev., vol. 69, no. 1, pp. 35–52 (2000); X. Wang et al., Thin Solid Films, vol. 390, pp. 130–133 (2001); and M. Okai, T. Muneyoshi, T. Yaguchi and S. Sasaki, Appl. Phys. Lett, vol 77, pp. 3468 (2000)). Some of these reports demonstrate the production of SWNTs using arc and laser techniques. Carbon vapor deposition using transition metal catalysts tends to produce MWNTs as a main product instead of SWNT. Catalysts containing iron, cobalt, or nickel, for example, have been used at temperatures of about 850° C. to 1200° C. to form MWNT (see U.S. Pat. No. 4,663,230).
There has been some success in producing carbon SWNTs from the catalytic cracking of hydrocarbons. Rope-like bundles of SWNTs were generated from the thermal cracking of benzene with an iron catalyst and sulfur additive at temperatures between 1100–1200° C. (Cheng, H. M. et al., Appl. Phys. Lett. 72, 3282 (1998); Cheng, H. M. et al., Chem. Phys. Left. 289, 602 (1998)). These SWNTs are roughly aligned in bundles and woven together, similar to those obtained from laser vaporization or electric arc method. The use of iron and at least one element chosen from Group V (V, Nb and Ta), VI (Cr, Mo and W), VII (Mn, Tc and Re) or the lanthanides has also been proposed (U.S. Pat. No. 5,707,916).
SWNTs have also been produced from the disproportionation of carbon monoxide (CO) with a molybdenum catalyst supported on alumina heated to 1200° C. (see Dai, H. et al., Chem. Phys. Lett. 260, 471 (1996)). From the reported electron microscope images, molybdenum metal appears to attach to nanotubes at their tips. The reported diameter of SWNT generally varies from 1 nm to 5 nm and seems to be controlled by the Mo particle size.
Generally, carbon SWNTs are preferred over MWNTs because SWNTs have fewer defects and are stronger and more conductive than MWNTs of similar diameter. There remains a need for methods of producing carbon nanotubes, and in particular, single-walled nanotubes.
Therefore, an object of the present invention is to provide a method for producing carbon nanotubes.
Another object of the present invention is to provide a method for producing carbon single-walled nanotubes (SWNTs).
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.